Frequency-shift keying system



E. W. PAPPENFUS FREQUENCY-SHIFT KEYING SYSTEM Oct. 23, 1951 Filed May 25, 1949 FIG. 3a;

FIG. 2.

FIG. 3.

R m a m 5 w m m P a ma V V a V w w 1 G4 V H 4 v M P G b W v Emvssr w. PA PPENFUS BY 7 z Vip ATTORNEY Patented Oct. 23, 1951 UNITED STATES PATENT OFFICE FREQUENCY-SHIFT KEYING SYSTEM Application May 25, 1949, Serial No. 95,217

Claims. 1

This invention relates to Wave-signalling systems, and more especially it relates to systems of the keyed frequency-shift kind.

Various keyed frequency-shift transmitter arrangements have been proposed and used heretofore, employing a so-called reactance tube whose anode-cathode current conductivity is used to control the capacitive or inductive reactance of a tuned oscillatory circuit. Thus, in one arrangement that has been widely used, a relatively low frequency oscillator, for example at 200 kilocycles per second, has its generated frequency shifted by a reactance tube on either side of the mean or center frequency of the oscillator, which shifted frequencies represent respectively mark and space telegraph signals. While such an arrangement has been found satisfactory where there is a fixed range of frequency shift on either side of the center frequency, nevertheless, where the total shift requires substantial increase beyond that fixed amount, it has been difficult to produce the mark and space frequencies symmetrically and with equal frequency deviations from the center frequency. In fact, with such systems, comparatively elaborate center frequency adjustment and monitor means may be. required. In an effort to overcome this disadvantage, it has been proposed heretofore to use a pair of reactance tubes connected for push-pull operation, one

tube controlling the capacitive reactance and the other tube controlling the inductive reactance'.

However, since the signal keying potentials are usually in the nature of direct current voltages, considerable difficulty has been encountered with such push-pull arrangements,

.among which difficulties have been the instability and non-linearity of the frequency-shift response, and the requirement for using phase inversion at the input to the push-pull tubes.

Accordingly, one of the principal objects of this invention is to provide a frequency-shift arrangement having the advantages of the prior single reactance tube arrangement, and certain of the advantages of the prior push-pull react- .ance tube arrangement, but without the abovenoted disadvantages of the prior arrangements.

-'nal--controlled variable direct current potential applied to its input control.

Another feature relates to a frequency-shift arrangement employing a pair of grid-controlled electron tubes whose grids are excited respectively in leading and lagging phase quadrature with respect to the center frequency of the oscillator; and one of the tubes of the pair has applied to its input circuit a steady direct current voltage, while the other tube of the pair has applied to its input circuit the different direct current keying voltages representing respectively mark and space telegraph signals.

Another feature relates to a frequency-shift arrangement employing a pair of grid-controlled electron tubes whose grids are excited in opposite phases from the frequency-shift oscillator itself, one tube being excited in quadrature leading phase with respect to the oscillator and the other tube being excited in quadrature lagging phase with respect to the oscillator; in conjunction with means for applying direct current keying voltages to only one tube of the pair.

Another feature relates to a frequency-shiftable oscillator employing a pair of frequencyshift control tubes, whereby symmetry and linearity of the frequency shifts on either side of a center frequency, are obtained without requiring special phase inversion circuits between the direct current keying source and the input electrodes of said pair of tubes.

Another feature relates to a frequency-shiftable oscillator employing a pair of reactance control tubes feeding a common tube whose output phase with respect to the oscillator can be rendered either quadrature-leading or quadrature-lagging while applying direct current keying voltages to the input of only one of said pair of tubes.

A still further feature relates to the novel organization, arrangement and relative interconnection of parts which cooperate to provide an improved and stable frequency-shift keying transmitter.

In the drawing, which represents, by way of example, one preferred embodiment,

Fig. l is a schematic Wiring and block diagram of a frequency-shift transmitter according to the invention.

Figs. 2, 3, 3a, 4, 5 and 5a, are respective vector diagrams explanatory of the operation of Fig. 1.

Referring to the drawing, the block I represents any well-known source of direct current keying control voltages, the: keying portion of which is schematically represented by the twoposition switch 2. When the arm 3 of this switch is on contact 4, zero or ground potential is applied to conductor 5. When the switch arm is on contact 6, a positive potential of E volts is applied to conductor 5. Associated with the source I are two control tubes VI, V2. These tubes may be of any well-known grid-controlled vacuum tube type, whose plate resistances or conductivities are respectively controlled by the direct current voltages applied to their control grids. The conductor 5 is connected through a suitable grid leak resistor l to the control grid 8 of tube VI. Thus, the grid 8 has applied thereto either a positive potential E representing, for example, a mark telegraph signal, or zero volts representing a space telegraph signal. The control grid 9 of tube V2 however, is connected through a suitable grid leak resistor ID to a source of positive direct current potential I I, which is whereby the voltage waves at the conductor 23 lag in phase by 90 the phase of the waves at conductor 36. 'I'he'vector and phase relation of the waves appearing respectively at point 34 and at the grids of tubes VI and V2, is illustrated in Fig. 2. From this figure it will be seen that the grids 8 and 9 are excited in 180 or opposite phase, and the phase of the waves at grid 8 leads the phase of the oscillations from tube V4 by 90, whereas the phase of the waves at grid 9 lags in phase by 90 the phase of the said oscillations from tube 4. The tubes VI and V2 have a common plate load resistor I4 to produce thereacross a voltage 'wave which is the vector resultant of the voltage waves at plates I2 and I3. If each tube has the same gain, that is if the direct current Voltages applied to their grids are equal in magnitude,

arranged so as to have a positive value which is equal to E/2. The anodes or plates I2, I3, of both tubes are connected through a suitable plate load resistor I4 to a suitable positive terminal I5 -on the usual direct current plate power supply.

The cathode I6 and the cathode I? are returned to ground-through a common cathode bias resistor I8 which may be shunted by a suitable by-pass condenser I9. The ungrounded end of resistor I8 is connected through an adjustable contact arm to a resistor 2 I, connected directly between the cathodes I6 and II.

By this manner of connecting the cathodes together and through the adjustable contact 29 and resistor I8, a certain amount of phase inversion is automatically efiected in the common output of tubes VI and V2. For example, when a positive potential E is applied to grid 8, the I. R. drop through resistance I8 biasses the cathode I! in such a direction as to tend to reduce the plate current of tube V2. Likewise, when the grid 8 has zero voltage impressed thereon, the plate current of tube V2 produces a drop across resistance -I8 which tends to reduce the plate current of tube VI.

The grids 8 and 9 are also arranged to be continuously excited by the carrier frequency, for example 200 kilocycles per second supplied respectively over conductors 22, 23, and through re- "spective condensers 24, 25. This carrier frequency applied to conductors 22 and 23, is derived from the oscillator tube V4 which may be of any wellknown grid-controlled vacuum tube type, provided that suitable feedback circuits cause it to generate normally the desired center frequency of for example 200 kilocycles per second. Merely by way of example, the tube V4 is shown as an oscillator of the Hartley type, wherein the high frequency inductance 26 has a portion'E'I connected through condenser 28 between the control grid 29 and the cathode 30. While another section 28 of this inductance is connected between cathode 39 and the negative or grounded terminal of the conventional direct current plate power supply whose positive terminal 3I is connected through the inconsist of any well-known combination of shunt condensers 53,44, and series resistor 45,, 46,

then the resultant wave voltage across resistor I4 is zero. In order to facilitate equalizing the gains of both tubes, the arm 20 can be adjusted on the resistor 2|. By means of adjusting contact arm onv potentiometer resistor 2I, the magnitude of leading lagging plate voltage V2P and V IP at plates I2 and I3 can be adjusted to equal magnitude as shown in Fig. 4 when a voltage equal to 13/2 is applied to grids 8 and 9 of VI and V2. It should be observed that in the usual way, the plate voltage of each of the tubes VI, V2, is out of phase with the excitation of its respective control grid. This accounts for the fact that in Fig.4 the voltage VZP leads the oscillator voltage V4, while the plate voltage VIP lags the oscillator voltage V4. When the system is thus adjusted; the voltage developed across the resistor I4 is zero so that no radio frequency control voltage is applied to the grid 49 of tube V3, and the oscillator V4 generates at its selected center frequency. It will be observed that the oscillator V4 is provided with the usual tuning condenser 5'! to tune the oscillator to the'desired center frequency, and the tube V3 being coupled to the tuned oscillatory circuit 265I through condenser 56, forms an effective control over the reactance of said tuned oscillatory circuit. In other words, tube V3 forms a so-called reactance tube for the said tuned circuit. The plate load resistor I4 is coupled into condenser 41, and thence through adjustable p0- tentiometer resistor 48 into the control grid 49of another vacuum tube V3. The adjustable poten tiometer resistor 48 serves as a gain or voltage control determining the proportion of radio frequency voltage across resistor I4 which is fed to the control grid of V3. In this wa the frequency shiftefiect of the system can be adjusted.

The cathode 50 of V3 is suitably bypassed by the cathode resistor 5| and the'usual shunt condenser 52, so as to operate upon the linear portion of the grid-voltage plate-current characteristic curve of the tube. For this purpose, the plate 53 is connected through a suitable load resistor 54 to the positive terminal 55 on the direct current plate power supply. The load resistor 54 is connected through a suitable condenser 56 to the point 34 in the tuned circuit of the oscillator.

vWhen the system has been thus initially'balanced to cause oscillator V4 to generate at the center frequency, it is in readiness to receive the keying control voltages from the source I. As above pointed out, the grid 8 is arranged to have either one or the other of two direct current voltage conditions applied thereto, namely zero voltage or voltage +E, while the grid 9 is arranged to' have a steady positive voltage E/2 applied thereto. Assuming for example that a space signal, represented by zero volts, is applied from source I togrid .8, it will be seen. that the gain of tube VI is less than the gain of tube V2, to that the vector relations. between the two volta waves applied to resistor 14 are as represented by the diagram of Fig. 3. Consequently, there is applied to the grid 49 a controlyoltage repr sented by the difference between the voltages V2? and- VIP (Fig, 3).. As a result, there appears tiplied by the amplification factor .of tube V3, which is effective throughcondenser 56 to. cause the oscillator .WLto change its frequency to the lower or space .frequency,.f.or example 1995 kilocycles'per second. On the other hand, when .a .mark signal voltage +E is applied to. grid 8, the corresponding vector relations in the plate voltage waves of tubes Vi and V2, are illustrated in Fig. 5. Thus the resultant differential voltage at the plate of tube V3 is then represented by the vector diagram of Fig. 5a. In this latter diagram, it will be seen that the resultant control voltage V3;- leads the oscillator voltage V4, and is of a predetermined magnitude, so that it shifts the frequency of oscillator'V4 to its upper shift frequency representing mark, for example 200.5.

Thus it will be seen that an equal increase and decrease in frequency from the center frequency is brought about by applying +E volts or zero volts to grid 8, while maintaining the grid 9 steadily biassed with a voltage of E/ 2.

It has been found that with this arrangement, it is possible to eliminate the instability which is usually encountered where two tubes are operated in push-pull relation with the grids biassed by direct current keying potentials. Furthermore, by reason of the particular manner of connecting the cathode return circuits of tubes VI, V2, and the manner of exciting their grids, phase inversion of the carrier voltage occurs without the necessity of inverting the direct current keying voltages. The shifted frequency carrier can then be coupled through a condenser 53 to a suitable transmission channel such as a radio transmission channel, a wire line transmission channel, or the like.

While in the circuit disclosed in the drawing, 1

the various tubes are shown as of the triode or single grid type, it will be understood that they may be of any plural grid type such as tetrodes, pentodes, or the like. Likewise, while the tubes VI and V2 are shown with their electrodes en closed herein, it will be understood that various changes and modifications may be made therein Without departing from the spirit and scope of oscillator, means to excite the grids of said control tubes from said oscillator in respective leading and lagging electrical phases with respect to the oscillator phase, and means to apply to the input of one control tube direct current keying-- potentials to shift the oscillator frequency.

2. A frequency-shift arrangement, comprising a variable frequency oscillator source whose fre:- quency is to be shifted between predetermined limits, a pair of frequency control tubes for said oscillator, means to excite the grids of said control tubes from said oscillator and respectively in mutually opposite electrical phase and respectively in leading and lagging electrical phases with respect to the oscillator phase, and means to apply to the input 'ofone control tube direct current keying potentials to shift the oscillator frequency. r 3, A frequency-shift arrangement, comprising a variable frequency oscillator sourcewhosesfrequency is to be shifted bet veen predetermined limits, a pair of frequency control tubes. for said oscillator, means to excite thegrids of said tubes under control. of said oscillator with respective quadrature leading and quadrature lagging B150? trical phases with respectto the oscillator-electrical phase, and means to apply direct current control potentials to the grids of said tubes to correspondingly shift the oscillator frequency.

4. .A frequency-shift arrangement. according to claim 3, in which means are provided for applying a steady direct current potentialto the grid of one tube andior applying a controllable direct current keying potential to the .gridof the other tube. x

5- A frequency-shift arrangement accordingto claim 4, in which said steady-direct currentapotential is approximately one-half the difference between the said controllable direct current key-' ing potentials.

6. A frequency-shift arrangement, comprising an oscillator whose frequency is to be shifted between predetermined limits, electron tube means having a pair of control grids, a common output circuit for said tube means, means to excite one of said control grids from the oscillator through a phase-shifting network, means to excite the other control grid from the oscillator through another phase-shifting network, said networks being mutually proportioned so that the excitations of said grids are respectively in quadratureleading phase and quadrature-lagging phase with respect to the oscillator phase, means to apply direct current gain control potentials to said grids to develop in said common output circuit of said electron tube means a resultant voltage proportional to the diiference in magnitude of the potentials applied to said grids, and means to apply said resultant voltage to shift the frequency of said oscillator in accordance with the difference between said applied direct current potentials.

'7. A frequency-shift arrangement according to claim 6, in which the direct current potentials applied to one of said control grids var es between predetermined lower and upper limits corresponding to keying signals, and the direct current potential applied to the other of said control grids is one-half the difference of the voltages applied to said one of said control grids.

8. A frequency-shift arrangement according to claim 6, in which said electron tube means also includes a pair of cathodes and a pair of anodes, each cathode and anode cooperating with a corresponding one of said control grids, and a common output circuit for said anodes for developing said resultant voltage.

9. A frequency-shift arrangement according to claim 8, in which means are provided for applying said resultant voltage to the control grid of a reactance control tube, and circuit connections for applying the variable plate resistance of said electron tube to correspondingly shift the oscillator frequency.

10. A-frequency-shift arrangement according -to claim-9, in which said reactance tube has its control grid connected to the output circuit of said electron tube means through a potentiometer for the purpose described.

11. An arrangement. for controlling the frequency of an oscillator, comprising a reactance tube coupled to the frequency-determining portion'of said oscillator, electron tube means for controlling said reactance tube said electron tube means comprising a pair of control grins having a common output circuit and individual input circuits connected respectively to said grids, means to excite the input circuits of said tubes from said oscillator, means to phase-shift the excitation of one input circuit to quadratureleading phase with respect to the oscillator,means to phase-shift the excitation of the other input circuit to quadrature-lagging phase with respect applied to the other grid of the pair is arranged. to be varied to control the oscillator frequency.

13. An oscillator control arrangement according to claim 11, in which the gain control potential applied to one grid of the pair is adjustable between predetermined limits, and the gain control potential applied to the other grid of the pair has a magnitude approximately equal to the mean between said limits.

. 14. An oscillator control arrangement according to claim 11, in which said electron tube means comprises a pair of grid-controlled electron tubes having their cathodes returned to ground through a common cathode load resistor.

15. An oscillator control arrangement according to claim 11, in which said electron tube means comprises a pair of grid-controlled electron tubes having their cathodes connected together through a potentiometer-resistor, the contact arm of said potentiometer being connected to ground through a cathode-load resistor which is common to both tubes.

ERNEST W. PAPPENFUS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

